It generally follows a suspension crystallization, which is a very effective and inexpensive process for achieving a high purity of a desired chemical compound. What is utilized here is the fact that impurities are substantially displaced from the crystal lattice and remain in the mother liquor as crystals grow in a liquid. Even in a one-stage crystallization process, highly pure crystals of the desired compound are therefore obtained. If required, the suspension crystallization can be performed in several stages.
A crucial step which has a critical influence on the purity of the crystallized target product is the removal of the highly pure crystals from their mother liquor, which comprises the impurities in enriched form and the uncrystallized fractions of the target product, by a solid/liquid separation process. This separation process can proceed in a plurality of stages, and a so-called wash column is often used at least in the last stage. However, the wash column may also be the only separating stage. It essentially has the task of separating the comparatively pure crystal phase from the comparatively contaminated mother liquor.
Wash columns are likewise known from the aforementioned prior art. They comprise a generally cylindrical wall which delimits a process chamber. Frequently connected upstream of the process chamber is a distributor chamber, into which the crystal suspension to be separated in the wash column is fed. On its way from the distributor chamber into the process chamber, the crystal suspension is distributed substantially uniformly over the cross section of the process chamber. In the process chamber, a relatively dense crystal bed is obtained by mother liquor withdrawal and is conveyed through the process chamber (this can be done from the top downward or from the bottom upward). A melt of the molten crystals themselves is passed through the crystal bed in countercurrent as wash liquid.
For the formation of a crystal bed, different methods are useful in principle. In wash columns which work with gravity, the crystal suspension is introduced into the column from the top, the crystal bed forms in a sedimentation process and it is conveyed in the conveying direction only through the action of gravity.
The use of such columns is excluded from the process according to the invention, since generally no defined wash front is formed therein, especially when they are provided with a stirrer for part of their height (cf. FIG. 1).
The process according to the invention is accordingly restricted to processes in wash columns with forced conveying of the crystal bed (a comprehensive description of the different wash column types can be found, inter alia, in Chem.-Ing.-Techn. 57 (1985) No. 291-102, in Chemical Engineering Science Vol. 50, No. 17, p. 2712 to 2729, 1995, Elsevier Science Ltd., in Applied Thermal Engineering Vol. 17, No. 8-10, p. 879-888, 1997, Verlag Elsevier Science Ltd., and in the literature cited in the aforementioned literature references and also in DE-A 102 45 164, DE-A 102 11 686, DE-A 101 49 353, WO 03/041832 and WO 03/041833).
In wash columns with forced transport (or conveying) of the crystal bed, a conveying force other than gravity acts in the conveying direction (or transport direction) of the crystal bed.
In principle, wash columns with forced transport of the crystal bed are divided into pressure columns (also known as hydraulic wash columns or hydraulic columns) and into mechanical columns. In the case of the pressure columns, the crystal suspension is conveyed into a wash column under pressure (for example by pumping and/or hydrostatic head). The liquid flow imposed by the feed column pressure then ensures compaction of the crystals to a crystal bed (cf. FIG. 2) and its conveying (the hydraulic pressure is typically from 0.1 to 10 bar, frequently from 1 to 5 bar). The mother liquor generally flows out of the wash column through filters (beyond the filter, the pressure may be standard pressure, reduced pressure or superatmospheric pressure). The recycling of a portion of the mother liquor enables the control of the transport force (control stream).
In contrast, mechanical wash columns comprise a mechanical forced conveying device for the crystals. In the simplest case, this may be a semipermeable plunger which is permeable to the mother liquor but impermeable to the crystals in the suspension supplied (cf. FIG. 3) and through whose shifting the pressure to densify and convey the crystal bed is generated.
The densification to a crystal bed and its conveying can, though, also be effected by removing the mother liquor through filters and mechanically transporting the crystals from the filter to the crystal bed by means of a rotating conveying element (for example screws, stirrers, helices or spirals) (cf. FIG. 4). The filters may also be integrated into the rotating conveying elements. Beyond the mother liquor outlet, the pressure here too may again be standard pressure, reduced pressure or superatmospheric pressure.
The crystal bed in the wash columns with forced transport of the crystal bed to be used in accordance with the invention has a so-called buildup front at which crystals of the crystal suspension introduced continuously add on. The buildup front thus indicates the transition from the suspension to the crystal bed and is characterized by a relatively abrupt rise in the crystal content in the suspension. In hydraulic wash columns, this buildup front is also referred to as the filtration front.
At the opposite end of the crystal bed to the buildup front, a kind of rotor blade (for example slotted rotating knife blade) or scraper is usually arranged, which continuously removes crystals from the crystal bed. The continuous addition of crystals at the buildup front on the one hand and the continuous removal of crystals at the opposite end of the crystal bed to the buildup front on the other hand defines the transport direction of the crystal bed (it may either point from the top downward or from the bottom upward). The crystals removed from the crystal bed are, if appropriate after they have been resuspended in pure melt, melted by heat transfer. A portion of the melt is removed as pure product stream and another portion of the pure melt is recycled as wash liquid against the transport direction of the crystal bed at its opposite end to the buildup front into the process chamber. Typically, the wash liquid is at melting point temperature.
The melting of a portion of the crystals can, though, also be undertaken immediately within the wash column (for example by means of corresponding installed apparatus for heating at the opposite end of the process chamber to the buildup front).
In that case, likewise only a portion of the melt obtained is withdrawn from the column. The other portion rises as wash melt.
The conveying of the pure melt in the opposite direction to the conveying direction of the crystal bed virtually forces the crystal bed saturated with mother liquor into the pure melt, and the mother liquor is in fact forced back to a certain degree by the pure melt within the crystal bed.
In the steady state, the result of this process is a wash front at a defined height in the crystal bed, which is defined as that point in the process chamber in the wash column where the highest temperature and concentration gradients occur (i.e., in the wash front, the temperature, for example, jumps, above and below the wash front, essentially constant temperatures are present). Roughly speaking, pure melt and mother liquor adjoin in the wash front. The region from the wash front to the buildup front is referred to as the mother liquor zone and the region from the wash front up to the opposite end of the crystal bed from the buildup front is referred to as the pure melt zone. The position of the wash front can be adjusted by controlling transported mass flow rate of crystals and opposing pure melt flow rate. It is frequently the case that the washing action becomes better with increasing length of the pure melt zone. Normally, the wash front has a longitudinal dimension (at right angles to the cross section of the wash column) of ≦100 mm, generally ≦60 mm and usually ≦40 mm. The latter longitudinal dimensions apply especially when the crystal bed, in contrast to EP-A 098 637, is completely unstirred. The cross section of the process chamber of the wash column may be circular, oval or angular (for example regular polygonal).
As the material for the wall delimiting the process chamber of the wash column, WO 03/041832 recommends the use of metal. The metals used may, according to the substance to be purified, be metals of different kinds. The metals may be pure metals or else alloys, for example carbon steels, iron-base alloys (stainless steel, for example with Cr/Ni addition) or nickel-base alloys (e.g. Hastelloy qualities). When the crystals to be removed in a purifying manner are those of acrylic acid (when acrylic acid is the target product), the preferred wall material of the wash column is stainless steel, especially stainless steel of DIN material No. 1.4571 or 1.4541, or stainless steel similar to these stainless steels with regard to the alloy elements. The thickness of the metal wall delimiting the process chamber is appropriately from 3 to 30 mm, frequently from 4 to 20 mm and usually from 5 to 15 mm, especially in the case of stainless steel.
A disadvantage of using metal as the material for the wall delimiting the process chamber of the wash column is, however, the high thermal conductivity of metals (at 300 K, the thermal conductivity of unalloyed steel is, for example, 50 W/m·K, that of stainless steel 21 W/m·K and that of aluminum 237 W/m·K, while the corresponding thermal conductivity of glass is only 1.0 W/m·K).
The aforementioned is disadvantageous in that the melting point of a pure substance is at a higher temperature than the melting point of the same substance but comprising impurities (freezing point depression). The consequence of this fact is that the temperature in the mother liquor is normally below the temperature in the pure melt zone. According to the impurity content of the mother liquor, this temperature difference can be up to 15° C. and more, frequently from 4 to 10° C. and, in the case of only a low impurity content of the mother liquor, from 2 to 4° C.
Owing to the high thermal conductivity of metals, this leads to the fact that heat is removed through the metal wall surrounding the process chamber from the pure melt zone at a higher temperature to the mother liquor zone at a lower temperature. The result may be undesired crystallization over the length of the pure melt zone on the side of the metal wall facing toward the process chamber, which reduces the throughput through the wash column owing to increased frictional losses or increases the pressure drop.
As a remedy, WO 03/041832 recommends introducing a controlled specific heat flow from the outside through the metal wall into the process chamber of the wash column at least over the length of the pure melt zone, specific heat flows of from 10 to 50 W/m2 being recommended as very particularly preferred.
As a realization of such a heat flow input which is particularly favorable from an application point of view, WO 03/041832 recommends surrounding the wash column as such with an outer housing and keeping the air present between outer housing (which is permeable to the ambient air) and metal wall at a temperature above the temperature of the melting point of the pure melt withdrawn from the wash column by means of heating. The difference between the two temperatures may be up to 20° C. or more.
In the case of acrylic acid, methacrylic acid, N-vinylpyrrolidone or p-xylene crystals, such a procedure has, however, been found to be disadvantageous, since, according to D'ANS•LAX, Taschenbuch für Chemiker und Physiker [Handbook for Chemists and Physicists], Springer-Verlag, Berlin 1964, Vol. II, Organische Verbindungen [Organic Compounds], the melting points of the aforementioned compounds as a pure substance are in the range from 12 to 16° C.
In other words, the temperature of the suspension in which the crystals of the aforementioned relevant compounds normally occur in the corresponding suspension crystallization (and hence the temperature of the mother liquor zone in the wash column) will generally be ≦10° C., and the temperature of the ambient air present in the outer housing will typically be ≧17° C.
This is disadvantageous in that, for example, the dew point of air at a temperature of 18° C. and a relative air humidity of 85% is 15.4° C. (even in the case of a relative air humidity of the aforementioned air at 18° C. of only 65%, its dew point at 11.3° C. is still above 10° C.). In the case of a relative air humidity of 95% of the air at 18° C., its dew point is 17.2° C. At an air temperature of 15° C. and a relative air humidity thereof of 95% (75%), its dew point is still 14.2° C. (10.6° C.).
The relative air humidity specifies how many percent of the maximum possible water vapor content at the particular temperature the air comprises. The dew point temperature (the dew point) is defined as the temperature at which the current water vapor content in the air is the maximum (100% relative air humidity). In other words, when the temperature goes below the dew point temperature (the dew point), a portion of the water vapor present in the air condenses out.
Therefore, when the described recommendation of WO 03/041832 is followed, water vapor will generally condense out of the ambient air on the metallic outer surface of the wash column at least in the region of the mother liquor zone. Such condensate formation is disadvantageous for various reasons. Firstly, condensate dripping off the wash column is capable of undesirably simulating leakage of the wash column. Secondly, the aqueous condensate can lead in an undesired manner to corrosion phenomena (for example regarding the drive of the rotor blade for the continuous removal of the crystal bed), and the heat of condensation released in the condensation can finally impair the homogeneity of operation of the wash column.